Morse is the one in front, Lewis is the one behind.
They did a good few series of Morse and then killed him off in the last one. It was fairly standard crime writing, well portayed and acted and with some wonderful filming moments. But then as a spinoff they decided to make Lewis into an Inspector and give him his own show, producing something fairly visionary in the process.
In 'Morse' Lewis is a "steady copper". He's from oop north (not sure it's ever said where but I'm guessing Newcastle). He plays the part of the sidekick that you can relate too, in the same club as Watson and Hastings, while Morse does the brilliant stuff. In 'Lewis' however, Lewis is still a steady copper sidekick, except now he's in charge of the cases. In order to retain the northerner/posh-uni dynamic they give him a Sargent from Cambridge, who does the buisness of knowing odd facts and strange details. Lewis still works out the cases, but unlike Morse (and indeed Sherlock Holmes and even Poirot) he doesn't pull random bits of polished knowledge out of nowhere. He just puts everything together, still in a fairly slow sidekicky manner.
Lewis is on the right and his Sargent sidekick Hathaway on the left.
The most amazing thing about this is that it works. Rather than being a distant insanely-intelligent detective Lewis is someone you can actually relate too. Sherlock Holmes is a force unto himself, Poirot uses deductive reasoning par excellent, but Lewis uses non of those tricks, just his knowledge, experience, and a normally intelligent mind.
That's not to say you can guess the end. You rarely can. But you can clearly see how Lewis gets there, and his experience plays a large part. Hathaway is the Morse-like figure here, intellectually brilliant, fold of crosswords, puzzles, full of random bits of knowledge whenever Lewis needs it, but he lacks the experience and the knowledge of human nature that Lewis has acquired. It makes the balance between them more even as well. With Morse and Lewis (as with Holmes and Watson) the poor sidekick has no chance to be better than the detective at anything. The message of the sidekick is "here is someone for you ordinary mortals to relate too, except obviously it's someone slow and a little dense because you will never be as amazing as the main character".
Lewis has faults as well, and not just the on-screen-harmless 'likes drinking' ones. He has prejudices and dislikes that affect his work (his wife died in a carcrash leading him to be prejudiced against suspects previously guilty of drink or drug driving) and Hathaway gently calls him up on these. Hathaway himself is a marvellous character, funny with dry wit, and amazingly acted. I like watching his face while other characters are talking, little smiles, or sudden small frowns (all very subtle) help to shape the character and the way he sees the world. Conventially, the Cambridge educated, smart sophisticated and intellectually brilliant Hathaway would be the main character, with his occasionally slow and awkward northern superior as the sidekick. But he isn't. Lewis is clearly and obviously the star.
The pacing is gorgeous as well. Once it starts each episode ramps up the pressure, slowly, then warms up slightly quicker and finally hits the last ~15 minutes with sheer fast-paced intensity. The cameras are used really wonderfully as well, with some filming moments which just make me squee at the screen (much to the confusion and amusement of my fiancé). Camera angles and shots are actually used for effect, rather than just to change the view.
But I think it really is the characters that get to me most. In the Holmes/Morse/Poirot school of detective writing the sidekick is just a prop, to hold up the brilliance of the detective. Lewis and Hathaway on the other hand, are truly a team; their faults and strengths compliment each other, and they get along well enough to gently tease about it. Lewis calls the shots, but it is more a partnership of equals than the brilliant detective with his sidekick running to keep up.
(And while I'm on the subject of detectives I must say I loved the recent Sherlock Holmes. It was let down badly by the villains, who were stereotyped-evil and far too unbelievable. Sherlock Holmes and Watson however went straight to my heart and I really hope they make better villains if they make another film).