Professor McGonagall: In the first film, McGonagall was portrayed by Maggie Smith. However, due to scheduling conflicts, Smith was unable to appear in the second film, and Fiona Shaw took over the role. Smith returned for the third film and continued in the role for the remainder of the series.
Professor Lupin: In the third film, Lupin was portrayed by David Thewlis. However, due to scheduling conflicts, Thewlis was unable to appear in the fourth film, and Timothy Spall took over the role. Thewlis returned for the fifth film and continued in the role for the remainder of the series.
Sirius Black: In the third film, Sirius Black was portrayed by Gary Oldman. However, due to scheduling conflicts, Oldman was unable to appear in the fourth film, and David Thewlis took over the role. Oldman returned for the fifth film and continued in the role for the remainder of the series.
Voldemort: In the first two films, Voldemort was voiced by Richard Bremmer. However, in the third film, Bremmer was replaced by Ralph Fiennes.