I've been conducting the musical Crazy for You over the past couple weeks and have been reflecting on the differences in conducting musical theater vs. opera, since at this point I've been fortunate enough to do many of each at both the collegiate and professional levels.
Certainly I think the hardest thing in musical theater is making the underscoring work properly - in other words the music that is performed while dialogue is in play. The timing can be particularly tricky depending on how consistent the actors are in delivering text, never mind the sets that are sometimes moving as well. Ending the music at exactly the right moment can present problems especially if safety bars are not utilized. In addition, it is difficult to keep the character of the music vibrant when the dynamic is very soft - the palatte becomes more narrow, you don't want to upstage the dialogue, and yet you want it to contribute to the mood of the moment with musical touches.
Opera has recitatives in virtually all periods that can be challenging, but the reality is if you can sing the music fluently and you have artists that aren't interested in throwing curve balls, provided you have technique, it's not that difficult to manage if you know what you want.
Another feature of musicals that I think can be tough, especially in dance shows, is to make fresh music that is subject to gravity, similar to ballet. You have to find an exact tempo that allows dancers to hit their marks, and this sort of precision can be difficult night in and out, since even a click of difference can make things fall apart. If it's quarter = 72, it can't go at 73! While opera requires a certain exactness, it tends to breathe a little more in the bulk of the repertoire.
Where musicals tend to have more flexibility is in their recitatives and in ballads. In those moments, it becomes more operatic in a sense, and the music making can be more present tense as it feels to me in much of opera.
One thing that makes opera easier for a conductor is the training of singers/actors. Generally, the folks in musical theater are not always as exact in their ability to interact with the gesture from the pit, whereas opera singers are drilled from day one. I honestly think the answer has something to do with power structure: in opera, artistic administrators in collaboration with conductors choose the singers, so even if you are a diva, you have to make these folks happy. In musical theater, casting agents and producers make these decisions, and sometimes of course, the creators of the show participate as well - but not the conductor in my experience both on and off Broadway. There's something to say for the power of hiring in the relationship of stage to pit!
One thing that tends to be more difficult generally in opera is the work a chorus must do. They usually have intricate things to sing, and keeping a good sound, good balance, precise diction, and proper character is really a challenge, even if they have been well prepared by a competent chorus master. In a musical, usually the flexibility that the choral numbers demand is not nearly as complex as what one finds in opera, and so to me, it seems a little easier.
The other thing in opera is language. I've conducted in Italian, Spanish, German, and French. I've conducted songs by Shostakovitch and Kurtag in Russian, but not an entire opera, and I haven't done anything in Czech yet - a lot of repertoire I would love to get to at some point if I can find time to work on the languages. The diction, the fundamental understanding of a language's idiomatic nature in music, and the sense of how the sound of the language fits with the sound of the music are all challenges that opera presents in a different manner than a musical, since all musicals I've done so far have been in English.
I find conducting staged productions to be the most three dimensional musical experiences a conductor can have. When you distill the difficulties each genre presents, everything truly boils down to a few truisms if a show will be successful musically from the conducting vantage point:
1) You have to know the score inside and out - especially the words and it has to sing you.
2) You have to have a fluent stick technique in the modern world if you want to it to be truly great.
3) You have to love what you're doing when you're doing it
4) You have to multitask between stage and pit
5) You have to understand the nature of the characters and be compassionate with them.
6) You have to be patient with the singers, but not too patient
7) You have to understand who has the power when decisions are made
8) You have to create as much face time as possible with everyone if a production is to gel.
9) You have to be efficient in rehearsal
10) You have to be flexible
Put all these into play and the feeling of pulling the reigns on an opera or musical production can be absolutely thrilling.