While watching an otherwise tedious movie my daughter wanted to watch, I amused myself by focussing on things of scientific interest featured in the film: the Segway the protagonist’s vehicles, the exotic bird that attacked him, and in particular, his nemesis, who had differently coloured eyes. I wondered if this really was a thing.
The actor playing the villain seems to normally have two bluish gray eyes, but other well-known actors (and of course non-celebrities) do have differently coloured irises, a condition known as heterochromia. It occurs about once in a hundred people, but it can be fairly common in certain dog, cat and horse breeds.
Eye colour depends on at least a few different genes that tell melanocytes in the iris where and how much eumelanin (dark brown) and pheomelanin (reddish) to produce. Blue eyes occur in irises because of a mutation that minimizes the amount of melanin produced. The blueness is the result of how the light gets scattered. Many Caucasian babies are born with blue eyes because they haven't produced melanin yet.
Heterochromia can be inherited or acquired. If it is inherited, most of the time it is benign, but it is probably worth getting checked. It could be some kind of syndrome with other complications, such as Waardenburg, Sturge-Weber, Parry-Romberg, or Horner.
An injury or illness affecting one eye can cause heterochromia. Actress Mila Kunis (whose mother is a physics teacher) has two differently coloured irises because of an eye problem. Glaucoma can also lead to heterochromia.
Heterochromia can be complete or incomplete, depending on whether the whole iris (heterochromia irides) is a different colour or only part of it is. The incomplete form can be central, around the pupil, like actress Jane Seymour (Dr Quinn, Medicine Woman) or segmental, in one part of the iris, like actor Simon Pegg (Scotty in the new Star Trek films).
This is starting to sound like a Hollywood/science gossip column. I was too embarrassed to include the links to the pages I had to look at for pictures of these celebrities with their differently coloured eyes. I don't personally know anyone with heterochromia, do you?