This article outlines the rationale for adopting a mixed methods approach within randomized controlled trials (RCTs) and explores challenges associated in doing so. Taking the example of the EASE Back feasibility and pilot study (Evaluating Acupuncture and Standard care for pregnant womEn with "BACK" pain: ISRCTN49955124), we detail why and how we operationalized a concurrent-sequential mixed methods research design. We present key findings from the exploratory research (focus groups and interviews) and explain how these were integrated with descriptive findings (a national survey of physical therapists) in order to inform and refine the design of the explanatory phase (the pilot RCT). We conclude with a discussion of lessons learned and implications for future research design and conduct.