Most Crime/Thriller Novels usually have a sudden, unexpected plot twist that puts the main character in a really hopeless situation.
One way to resolve this is by using a "Deus Ex Machina", that is a character that suddenly appears out of nowhere and has the solution. It could be that FBI Agent who is part of secret government project, the secretary of your enemy that gives you access to some codes, a long lost friend/relative who was believed to be dead, or the wife/child of the enemy.
I see the appeal of DXM resolutions as they don't require much preparation and are seemingly surprising, but nowadays I just find them annoying. But on the other hand, they do offer to introduce a completely new scenario for the rest of the story. That FBI Agent allows us to show a whole secret government facility, the lost relative has a hiding place somewhere, and the secretary/wife/child gives us an entry into the facilities of the enemy. Also it allows to bring in a ton of new people easily.
If on the other hand we introduce the saviour early in the story and have him/her resolve the hopeless situation, it feels a bit boring. It is hard to introduce new people and places if they have to be attached to the early-introduced sidekick, which leads to a reaction like "If you had an Uncle that can do that, why didn't you already go to him 150 pages ago when you first encountered that problem?". Also it then feels a bit like a journey, constantly watching 2 or 3 characters - a bit like the Sherlock Holmes novels that only had these two.
There is nothing wrong with Sherlock Holmes of course (Especially since it was written in the 1880's), but I'm trying to understand how people like Ken Follet manage to avoid this effect mostly.
So, what are ways to avoid a Deus Ex Machina resolution without throwing away the opportunity to introduce a new scenario for the rest of the story, and without being boring?