According to Genki, expressions of desire ( -たい sentences such as in the examples below) which use the particle を can also use the particle が interchangeably, and besides stating that, they give no further explanation. From what I could tell from searching around it seems that が appears to be a more natural choice (I could be wrong though). Does using を vs が imply a change in meaning? Is one particle preferred over the other in these types of sentences? Can they be used interchangeably 100% of the time or are there exceptions?
e.g. 映画が見たいです and 映画を見たいです, both of which should mean "I want to see a movie".
Using が casts the focus onto the object. Think of it in a similar manner to using an adjective—you're describing the state of the movie by saying you find the prospect of watching it desirable. Using を instead of が focuses on the action—you're still saying that you want to see the movie, but you're emphasizing that you want to see something, as opposed to that you want to see anything in particular.