Hospital emergency departments (ED) offer an opportunity to engage with large numbers of people who smoke to prompt cessation, although the acceptability of opportunistic intervention in this context has been questioned. This process evaluation study was embedded into the Cessation of Smoking Trial in the Emergency Department (COSTED) randomized controlled trial and sought to explore the context of intervention delivery within the ED. Qualitative interviews were conducted with participants and staff across six EDs participating in the COSTED randomized controlled trial. Interview data were thematically analyzed specifically exploring contextual influences. Data were triangulated with ethnographic observations. In participant interviews (N = 34), it was acceptable overall to receive a brief opportunistic smoking cessation intervention in the ED. Contextual factors are impacted at a range of levels. At the micro level participant views and experiences combined with staff tailoring were important. Being given an e-cigarette starter kit by a "credible source" helped to legitimize vaping for smoking cessation and gave confidence in personal ability to switch away from tobacco. At the meso level, adaptations to intervention delivery were made in response to the context of the ED. Stop smoking advisors (N = 11) had to adapt and deliver the intervention flexibly depending on space and clinical need. At the macro level, hospital policies supportive of vaping legitimized the approach. Smoking cessation outcomes reported in the main trial across sites were very similar because of the high credibility, acceptability, and flexible approach to delivering the COSTED intervention in the ED. Attending a hospital ED is the right time and place to receive smoking cessation intervention, even for those not motivated to quit. People are willing to receive intervention, and clinical staff are willing to support intervention delivery. Despite challenges, overall the context is helpful in supporting people to switch away from tobacco. The intervention, with flexible and tailored implementation, is adaptable to different ED contexts. This suggests that wider implementation across NHS Trusts of the effective COSTED intervention is feasible and will ultimately support smoking cessation for people attending EDs, who may not otherwise have sought support.