Acceptance of the virtual coach will be assessed using a Dutch translation of a set of six items that measure the evaluation of the coach in terms of satisfaction, usability, continuation of collaboration, relationship, preference for a program with or without coach and adherence to advice from the coach. The questionnaire has been used in other studies evaluating virtual agents (Bickmore et al., 2010; Provoost et al., 2020). Two example items are: "How satisfied were you with the virtual coach?" and "How easy was it talking to the virtual coach?". Answers are given on a 7-point Likert scale with different labels (e.g., from 1 (not at all satisfied) to 7 (very satisfied)).

Qualitative individual semi-structured interviews with participants will complement the quantitative data and provide more in-depth insight into acceptability, feasibility, preliminary effectiveness, and requirements for implementation of the Perfect Fit intervention.

Self-reported level of PA will be measured with a Dutch translation of the 3-item Godin-Shephard Leisure-Time PA questionnaire (GSLTPAQ; Amireault & Godin, 2015; Godin, 2011). Participants are asked how many times on average, during a typical week, they do strenuous, moderate or mild exercise for more than 15 minutes. The total score of the questionnaire is calculated by multiplying the number of times per week strenuous exercise by nine, multiplying the number of times per week moderate exercise by five, multiplying the number of times per week mild exercise by three, and taking the sum of this (i.e., (9 x times/wk strenuous) + (5 x times/wk moderate) + (3 x times/wk mild)). A total score of 24 or more is defined as 'active', a total score of 14 to 23 is defined as 'moderately active' and a score of less than 14 is defined as 'insufficiently active' (Godin, 2011).

Non-smoker self-identity will be assessed using seven items (Meijer et al., 2016), of which three are adapted from the Abstainer Self-Concept scale and three are adapted from the Smoker Self-Concept Scale (Shadel & Mermelstein, 1996) by replacing 'smoking' with 'not smoking'. In addition, the item "I would like to be a non-smoker" will be added (adapted from Tombor et al., 2013). All items are translated to Dutch and can be answered on a 5-point Likert scale ranging from 1 (strongly disagree) to 5 (strongly agree). Higher total scores indicate stronger non-smoker self-identity.

PA self-identity will be measured with a Dutch translation of the adapted version (Strachan et al., 2010) of the 9-item Exercise Identity Questionnaire (Anderson & Cychosz, 1994). PA identity will be measured rather than exercise identity since being physically active entails a broader array of physical activities (Strachan et al., 2010). As the Perfect Fit intervention aims to enhance PA by increasing the amount of steps, this includes not only sports/exercise, but also daily physical activities like household chores. An example item is: "I consider myself a physically active person". Answers are given on a 7-point Likert scale ranging from 1 (strongly disagree) to 7 (strongly agree) and the total score ranges from 9 to 63, with a higher score indicating stronger identity.

Smoking abstinence self-efficacy will be assessed with a single item (adapted from Loprinzi et al., 2015), translated in Dutch. Participants will be asked: "Overall, how confident are you that you are able not to smoke ?". Abstinence self-efficacy has been shown to be associated with smoking cessation (Spek et al., 2013). Answers are given on a 5-point Likert scale ranging from 1 (not at all confident) to 5 (extremely confident), with a higher score indicating higher abstinence self-efficacy.

To assess self-efficacy for PA the same Dutch single item (adapted from Loprinzi et al., 2015) will be used as for assessing smoking abstinence self-efficacy, but the item will be adjusted to PA behavior: "Overall, how confident are you that you are able to be sufficiently physically active?". An instruction will be provided before the question is presented, explaining the definition of PA (i.e., exercise, but also physical activities like walking to the supermarket) and the recommended levels of PA according to the WHO guidelines (World Health Organisation, 2022). PA self-efficacy has been shown to be associated with actual PA behavior (McAuley & Blissmer, 2000). Answers are given on a 5-point Likert scale ranging from 1 (not at all confident) to 5 (extremely confident), with a higher score indicating higher self-efficacy for PA.

The average number of steps per day will be measured with the smartwatch participants are wearing during the study and collected using the sensor data collector app. The sensor data will be collected during the Perfect Fit intervention.

Feasibility of the study will be assessed in terms of recruitment, response and consent rate using participant screening and inclusion logfile data. The following information will be logged: the time between the start of patient recruitment and the day on which the final participant is recruited (i.e., recruitment rate); proportion of interested participants who completed screening questionnaire (i.e., response rate); and number of eligible participants who consented to take part in study (i.e., consent rate).

To gain more insight in the effectiveness of different recruitment strategies, participants will be asked at baseline how they heard about the study. They can indicate which recruitment strategy was used (e.g., recruited directly by a researcher, via social media post). This will provide insight in effective recruitment strategies for future research, which is especially interesting in the case of participants with a lower SEP as they are often underrepresented in research.

Users' adherence to the study will be assessed using participant log data. Adherence will be examined in terms of the number of participants who completed the Perfect Fit intervention and the study, the number of participants lost to follow-up over time (dropouts), and the number of participants who do not drop out from the study, i.e. still filling in questionnaires, but who are no longer using the app (nonusers; Eysenbach, 2005). Adherence will be monitored during the full study duration by keeping a participant screening and inclusion logfile and will also be assessed with the earlier mentioned usage data and self-report question (see primary outcome 'Feasibility of the Perfect Fit intervention').

At the start of the post-intervention questionnaire, users will be asked if they completed the intervention (i.e., completed the final closing dialog with the virtual coach) or not. In case they did not complete the intervention yet, they are asked in which week of the execution phase of the Perfect Fit intervention they are at that moment. This way it can be assessed if participants fill out the post-intervention questionnaire while having completed the whole intervention or not. It also provides information on how many people complete the intervention in 16 weeks or less.

Qualitative individual semi-structured interviews with participants will complement the quantitative data and provide more in-depth insight into acceptability, feasibility, preliminary effectiveness, and requirements for implementation of the Perfect Fit intervention.

Participants will be asked for an intended timeline to quit smoking in the future. Answer categories are based on the stages of change of the Transtheoretical model (Prochaska & DiClemente, 2005) and include: within the next month (1), between 1 to 6 months from now (2), sometime in the future, beyond 6 months (3) or not planning to quit (4; Hummel et al., 2018). Participants who choose answer category four will be categorized as 'non-intenders'. This variable will be recoded so that higher scores indicate stronger intention to quit. Since intention to quit smoking is also one of the inclusion criteria, we expect that participants in the study are not likely to choose answer option 3 and 4.

Physical nicotine dependence will be assessed using the Dutch version of the Fagerström Test of Nicotine Dependence (FTND; Heatherton et al., 1991; Penfornis et al., 2021). The FTND consists of six items, of which four items are scored on a dichotomous scale (yes / no), one item is scored using four answer categories, and one item has an open answer format (asking participants about the number of cigarettes smoked per day). An example item is "Do you smoke more in the morning than during the rest of the day?". Total scores can range from 5 to 12, with a higher score indicating higher nicotine dependence.

The same question is used as for intention to quit smoking. First, an instruction will be provided before the question is presented, explaining the definition of PA (i.e., exercise, but also physical activities like walking to the supermarket) and the recommended levels of PA according to the WHO guidelines (World Health Organisation, 2022). Then participants are asked to indicate whether they intend to become more physically active. Answer categories are based on the stages of change of the Transtheoretical model (Prochaska & DiClemente, 2005) and include: within the next month (1), between 1 to 6 months from now (2), sometime in the future, beyond 6 months (3) or not planning to become more physically active (4; Hummel et al., 2018). Participants who choose answer category four will be categorized as 'non-intenders'. This variable will be recoded so that higher scores indicate stronger intention to become more physically active.

Participants will be asked if they made use of additional aids or support for smoking cessation or PA enhancement. Participants are asked if they have used any type of additional support (e.g., medication, booklets, websites, other apps, or treatment/support, like professional coaching, a sport challenge or acupuncture) next to the Perfect Fit intervention, and if so, what type of support. This will provide insight in other forms of support that were used and could influence the results. It will also provide insight in the applicability of the intervention (e.g., use as stand-alone or add-on intervention).

Lapses (defined as one time point of smoking again after the quit date, any quantity) and relapses (defined as multiple time points of smoking again after the quit date, any quantity) during the execution phase (i.e., phase which starts on the quit date) of the Perfect Fit intervention will be assessed using usage data. Usage data about cigarettes smoked after the quit date will be obtained in two ways. First, as part of the '(re)lapse dialog' with the coach. This dialog can be initiated by the user in case of (re)lapse to receive support from the coach to cope with this (re)lapse. During this dialog the coach will ask the user if it concerns a lapse or a relapse and how many cigarettes they smoked. Second, via the 'tracker' functionality of the NiceDay app. Users can use this functionality during the whole Perfect Fit intervention. Both ways of obtaining data on (re)lapses rely on self-report of the user.

Participants will also be asked if they smoke/use e-cigarettes. In case their answer to this question is 'yes', they will be asked if they smoke e-cigarettes with or without nicotine and how often. This is assessed as some people switch to e-cigarettes after quitting smoking tobacco.

Since the level of PA during work can differ between participants, participants will also be asked about the level of PA during their workday using a single item: "How would you describe your daily physical activity at work? Choose the answer that best suits your work activities:". The answer options are partly based on the Occupational Sitting and Physical Activity Questionnaire (OSPAQ; Reis et al., 2005) and the Global Physical Activity Questionnaire (GPAQ; Armstrong & Bull, 2006) and include: 1) Mostly sedentary (I sit almost all day and do little or no physical activity), 2) mild exercise (I am involved in light physical activities such as standing and occasionally walking, but not intensively), 3) moderate intensity exercise (I perform activities that require moderate physical exertion, such as fast walking, lifting light loads), 4) strenuous exercise (I mainly engage in physically demanding activities, such as heavy lifting), or 5) I am not working at the moment.