This usually happens in fantasy movies where a character drinks a potion or uses magic to disguise himself into another character.
For example, in Harry Potter And The Deathly Hallows, Hermione transforms into Bellatrix when breaking into Gringotts. But Helena Bonham Carter, plays Bellatrix as Hermione is disguise, with mannerism that Emma Watson brings to the character.
In Thor: Ragnarok, Loki disguises himself as Odin & rules Asagard for a while. Here, Sir Anthony Hopkins plays Odin as Loki in disguise, with mannerism like "Oh Shit" that Tom Hiddleston brings to the character.
In Thor: The Dark World, Loki disguises himself as Captain America when Thor breaks him out of prison. Here Chris Evans plays Captain America as Loki in disguise, with mannerisms like mischief that Tom Hiddleston brings to the character.
The question is how do these actors prepare for a situation like this. When they have to play their characters but as a different character in disguise and still keep the characters mannerism intact.
Do the Directors shoot a mock scene with the other actor in order for them to have a reference. ie, did Emma Watson do a mock-scene first for Helena to copy, did Tom do a mock-scene first for Sir Hopkins to copy. How does it work on-set?