Accessible Summary When a vulnerable adult is in police custody, they should have someone with them to help them understand what is happening. This person is called an appropriate adult (AA). Previous research has shown that the role of the AA is not always well understood. This study compares the views of both professionals and vulnerable adults. Adults with learning disabilities and mental health problems said looking after them and helping them communicate were the most important things an AA should do. Like other studies have shown, not all adults who should be provided with an AA in custody have had one. This may be because no agency has a statutory duty to provide one. This study recommends that AA services should try and engage more effectively with vulnerable adults. Abstract Background Police custody sergeants have a duty to secure an AA to safeguard the rights and welfare of vulnerable people detained or questioned by the police. This study focuses on the role of the AA in supporting vulnerable adults and seeks to examine what stakeholders would expect from an effective AA service. Methods This was a qualitative study of four AA services in England. Interviews were undertaken with 25 professionals, and two focus groups were held with service‐user groups (13 participants). Results There is disparity between the expectations of professionals, and service users, on what comprises an effective service. Professionals tend to prioritise the availability and response time of AAs, while service users prioritise their personal attributes and demeanour. Conclusions Professionals involved in commissioning and management of AA services should monitor whether the rights of vulnerable adults in custody are protected and better engage vulnerable adults beyond service delivery.