You wouldn’t get Christensen back just to be in the suit, so I imagine there will be some flashbacks. But I’ll throw my hat in for some Anakin-desert-hallucinations or something Tyler Durden-y. Obi-Wan “seeing” Anakin as he goes on his adventures.
Well they said he would be playing Darth Vader so I interpreted that as actually playing him in the suit. I can imagine they’re probably going to do both what you said and that.