First thing I know about The Tourist, is that it's a remake from a 2005 French movie Anthony Zimmer. So when watching The Tourist and I found myself asking why Interpol (so many of them) were after Angelina Jolie's character -and using this incredible technology, which must be expensive, which enables us to read a letter which has been burned to ashes- and is it necessary? for she is one of them anyway - I put that question aside for I haven't watched the French version.
Interpol is after her because she is the lover of Alexander Pearce, a gangster's private banker who has stolen from his boss; but the real reason Scotland Yard wants him is because Pearce owes tax money to the British government. They hope by tailing Elise (Angelina Jolie), sooner or later she will lead them to him. They have no idea how he looks like as he has undergone face surgery. When Elise leads them to Frank (played by Johnny Depp), the gangster believes that Frank is Alexander Pearce and is after him.