Normally, (maybe more than should be the case, even) movies-within-movies tend to get a pass if they don't look or feel quite convincing as they would were they the real thing. They don't look like the success the story wants us to believe they are. Sometimes they don't even look watchable. "Who cares?" we tell ourselves, "They're not the important part." And that may be true some of the time. One can see why, if a corner had to be cut somewhere, it would make sense to target this fluff, which usually doesn't have too much bearing on the overall plot.
Joel and Ethan Coen went the extra mile with Hail, Ceasar! It's a story of "Old Hollywood", the golden age of cinema and all that. The movies-within-this-movie are fluff, like most of their kind, but being that this is a movie about a movie studio, they're rooted a little deeper into the film's overall presence. They marinate it, give it its flavor. They're movies with catchy tunes, intriguing underwater sequences, and daring shootouts. They don't feel exactly like movies from the 40's or 50's, but they're close enough, and damn if they aren't great to look at, regardless. They're movies you catch yourself wishing you were watching instead, but while that kind of distraction is normally a hindrance, here you get the sense it was the idea here. It's the Brothers Coen playing around for their own amusement, and in spoiling themselves, they treat us a little bit, too.
Hail, Ceasar! gives us a day in the life of Eddie Mannix (Josh Brolin.) In form, Mannix is something of an errand boy for the studio at which he works, but in function, he's a PR magician. He's a babysitter to the eccentric actors and fussy directors, and when he has to be, he can serve as anything from an editor to a private investigator, all to keep everybody's name out of the mud. Today, Eddie's assignments are to ease a typecast cowboy (Alden Ehrenreich) into a new image, marry off a scandalously single-but-pregnant actress (Scarlett Johansson,) and pay off the ransom for the titular production's kidnapped star, Baird Whitlock, (George Clooney.)
Without giving too much away, the movie has the Coens' usual comedic staple: a mystery wherein our protagonists luck their way to a conclusion just as often as they get there through hard work. It's not quite as funny as Big Lebowski, but it has some great bits, my favorite being an increasingly frustrated exchange between Ehrenreich's character and his director (Ralph Fiennes.)
I'd like to stress though, that the flash beats out the substance in the case of this one, in the best of ways. The line between affectionate parody and flattering homage can be pretty blurry anyway, but Hail, Caesar!'s emulation of classic cinema makes that line a clear haze that may as well serve as the filter to give it that aged look.