Interviewing internal candidates can be difficult.  You already know the candidate, their capabilities, their personalities right?  Wrong.

Candidates had a life before they came to your company, and even if they are in your team, there may be experiences they have had which could be used which you don't know about.  They have a life outside of work, which may include training, volunteer work or family experiences which could be useful, but which you don't know about.  They have ambitions, desires, wants and needs which you may not know about but which affect the way they work now, and the way they may work in a new role.

Internal candidates should be interviewed in exactly the same way as external ones.  Take them through the standard question set, giving them the opportunity to tell you everything an external candidate would.  Put aside any perceptions you have, listening to the answers as if you don't know them.  Sometimes, the round peg in the square hole flourishes once a round hole is found.  If you have the opportunity to provide the round hole, and get a candidate who already knows your company, it's language and the way it works, why wouldn't you?