The Witcher 3

So here's the challenge with casting by appearance (whether that's by race, hair colour, height, whatever physical characteristics).

First you need someone who looks like the character in question, which is already dramatically narrowing your field of candidates. Then you need to actually find them. Then you need to ensure they're a good actor who can bring something fresh to the role - so you're down to a very small field indeed, perhaps already out of luck. Then the actor needs to be available and willing for the production. They need to have decent chemistry with the rest of the cast. And then you'll probably need agreement between all the key players - casting director, director, producer and perhaps even funder.

Ultimately - assuming there's no commercial shenanigans afoot and there's a degree of creative freedom involved in the casting - the most important factor is whether they're a good actor. If you get someone who looks like the fictional character as described or shown in source material, great - there are some brilliant casting directors out there who'll do their best to get the absolute best candidate! If not, also great! Physical appearance is an incredibly restrictive casting barrier for all the reasons described above, and ultimately something pretty much everyone won't particularly care about if the performance is good enough.

Naturally, it would be naive to assume everyone is cast based on acting talent alone That's completely not the case in reality. And there are plenty of circumstances where some kind of physical attribute is essential for the character in question - a production about race relations, realistic or fantastical, will obviously have far less leeway. In The Witcher, you're naturally not going to be casting a 6'5" person as a dwarf unless you have a healthy CG and effects budget There are also cases where extra efforts should be made if the role calls for an underrepresented minority actor - that's why there was such a backlash to Scarlett Johannson's casting in Ghost in the Shell, as at the time there were few lead roles for Asian-American actors (especially when the film works itself in weird loops to explain why she's not Asian). But for the most part what's key is that a performance captures the spirit and essence of the character in question, and brings something fresh and unique to the role. Restricting your casting to lookalikes means you're far less likely to get that.

How will all this play out in the context of The Witcher? Who knows *shrugs*. Personally I think the trailer looks a bit underwhelming mid-budget fantasy to me, and I'm not super passionate about the source material. I believe Cavill is a pretty bland actor... although that could suit the characterisation of Geralt in much of The Witcher 3