Real people in fiction

I have just seen the film "The Last King of Scotland" which features Idi Amin (played well by Forest Whitaker). But the main character in the film is a Scottish doctor who goes to Uganda to help and to have a good time. Did the person really exist? I don't think he did. We'd surely have heard the news of what happened to him there - pretty horrible stuff - and how he escaped to tell the story of the brutal regime.
So while I enjoyed the film, liked the acting, the setting in Africa, the various characters and their way of life, I was continually up against the feeling I always have in such dramas - I ask myself: "If he isn't a real person then what else in the film is untrue?"
I felt this when I saw a film about the four Irishmen who were unjustly imprisoned for years after being accused of bombing a city in England: the main character was in the same prison as his father.... But he never really was. So, I ask myself: "Should I believe any of this, is any of it true?"
Apparently the newish film "Hunger", about Bobby Sands, has similar incidents that are not true; so, again: "Can I believe any of it?"
The trouble is it's not fiction where I don't have to believe but where I suspend my disbelief; In these cases you can''t suspend your disbelief because they are telling you not to, but to believe.