I've been reading Sing Out, Louise!, a book published in 1993 that has numerous Broadway folks talk about what it's like to perform in musicals. Most of the names aren't huge stars, so we get a look at the less glamorous side of the Great White Way--getting fired, working as an understudy, having a show close out of town and so on. Very informative.
A Funny Thing Happened On The Forum (1962) was a surprise smash hit and ran for 964 performances, in large part due to Mostel's ability to make the audience accept the musical's makeshift plot devices and corny jokes.
Where did that come from?
I'm sure Mostel did a fine job, but he's not the only reason it ran. (He wasn't even the first, or second choice for the lead at the time.) Forum has become one of Stephen Sondheim's most-performed shows--it must have something to see so many productions without Mostel. There was even a revival on Broadway not long after Sing Out, Louise! came out that ran for two years.
And "makeshift" plot devices? The book writers, Larry Gelbart and Burt Shevelove, worked on the script for years to get it just right. A farce with music that last over two hours and holds audience interest doesn't just happen, it takes considerable planning. The jokes are corny? Perhaps the characters are venerable, the show being inspired by the farces of Plautus, but Gelbart and Shevelove give the characters good material, and talented clowns have been getting laughs with the script for over fifty years now.