The title of this post is a subject of recurring interest to me, combining as it does the World Championship (an old hobby of mine) with openings (an even older hobby), specifically the role of opening preparation in determining the World Champions. The issue of preparation is particularly relevant to my Chess960 Blog (a newer hobby).
The modern practice, where teams of analysts assist the World Champion or his challenger in dissecting and digesting the opening repertoire of the future opponent, is well documented in current chess literature. A good example can be found in 'From London to Elista' by Bareev and Levitov, which has inside looks at Kramnik's title matches in 2000, 2004, and 2006. Bareev was one of Kramnik's seconds for the first two matches.
I imagine that in preparing for the matches of 100 years ago -- Lasker vs. Tarrasch, vs. Schlechter, and vs. Janowski -- the players did little more than make a general survey of their future opponent's games, with the object of being familiar with positions likely to arise during the course of the match. When did the practice of serious preparation start? I suspect it was the 1927 Alekhine - Capablanca match. Alekhine was, after all, one of the first great practitioners of the science of openings. From 'The Soviet School of Chess' by Kotov & Yudovich (p.42):-
The titanic amount of work [Alekhine] put into studying strategy in preparation for the match with Capablanca in 1927 is well known. He studied the Cuban master's style down to its minutest details, discovered inconspicuous weaknesses in his apparently flawless playing, and defeated him.
It doesn't take much imagination to suppose that detailed scrutiny of Capablanca's openings was part of the study. As for Capablanca, his confidence in his own ability to solve all problems over the board probably convinced him that opening preparation was unnecessary. This made the 1927 match an unusual example of preparation by one side only. I assume this was also the case with the two Alekhine - Bogoljubov matches.
This changed with the two Alekhine - Euwe matches. Here were two adversaries both known for their meticulous study of the openings. From 'Extreme Chess' by Purdy, on the 1937 match (p.85):-
The Seconds: Each player was allowed a "second" who analyzed openings with him before and during the match, and who was also permitted to assist him with adjournment analysis! Alekhine's second was Erich Eliskases, the brilliant Viennese master; and Euwe's was Reuben Fine, the young American grandmaster and chess encyclopedia. Quite early in the match, Fine had to go to the hospital with appendicitis, so that Euwe lost his services.
Aside from the surprise indicated by '!' after 'assist him with adjournment analysis', the role of the two seconds was consistent with modern practice. Skip forward a full decade. For the next two decades after WWII, the World Championship was dominated by Botvinnik; Kotov & Yudovich again (p.133):-
One of the secrets of Botvinnik's success is his opening erudition. His knowledge of openings is as diversified as it is deep. He is remarkably fertile in inventing and playing new variations. His original, unexpected opening innovations have amazed the chess world time and again.
It's not difficult to imagine that opening study played an even deeper role in Botvinnik's preparation than it did for any of the pre-WWII matches. After Botvinnik, we have Fischer, Karpov, and Kasparov, none of whom were deficient in opening preparation.
Since much of the foregoing discussion is speculative -- 'I imagine', 'I suspect', 'probably', 'I assume', 'not difficult to imagine' -- I'll use this post as an anchor for more precise references. I've often found it curious how, once my attention latches onto a topic, supporting material crosses my radar with greater frequency than before.