It's always unfortunate when an actor/actress can't reprise a role in a series. Sometimes the character can't be written out because they're too important, so replacing the actor/actress is the only option. As a viewer, sometimes this works for me and sometimes it doesn't. I've only recently nailed down what dictates which way I'll go.
It boils down to two main issues:
- Do I know which character this is immediately? Has the writer made sure they're addressed by name the moment I first see them (or beforehand), or do they make me wait for confirmation?
- Does the character get character building time, as though they were a brand new character?
I've found that whether the person is better or worse at acting than the person they replaced isn't the main issue. As long as I can suspend my disbelief, and they meet the basics of acting skill, that doesn't matter.
The Dark Knight was one that didn't work for me. The new Rachel* was introduced in a "what do you mean you don't recognise her" way. I was left with that doubt about who she was, and that took me out of the film for the scene.
She also didn't get much character time in the early part, as though I'd still be attached to her from the events in the first film. But she wasn't the same person and the actions didn't transfer over. She needed something in this film to make me care. By the time the film ended, I was only just getting used to her.
Iron Man 2 took a different approach. The new Rhodey** is introduced in a courtroom scene, so his entry is announced. There's no doubt about his identity. There's also a little back-and-forth between him and Tony Stark, starting the process of getting attached to the character. It's interesting that you get a back view for this exchange, so you get to know him a bit before you're shown his face in detail (which highlights the physical differences).
The Rhodey transition was smooth, and after that initial "this is a different actor" moment, I didn't think anything more of it.
It's a pity more series that have actor/actress changes don't handle it well. It's not a good idea to keep an identical version of the script as though nothing has happened. Something has happened and the person on the screen has changed... the viewer needs a little help to get used to that.
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* Played by Maggie Gyllenhaal. Originally played by Katie Holmes in Batman Begins.
** Played by Don Cheadle. Originally played by Terrence Howard in Iron Man.