A senior developer is here to write good code but also to help younger developer. Thus attitude is key aspect of the candidate to consider and would ask :

- How do you handle a young developer who is making mistakes ? Do you take time to help him ? A concrete example would to give him/her a bad piece of code and ask him/her to peer-review it.

- Are you ready to give some talks about whatever your are good/expert at ?

- What is your attitude toward optimization (I would expect something like : don't optimize until it is correct and needed, but don't write the original code without performances in mind).

- Check if the candidate is stubborn. A senior grumpy stubborn developer can be a pain to work with.

Bonus: what is the job ?