Sometimes I need to get a feature into the project I'm working on, but the developer who owns the feature is too busy to implement it. A trick that seems to help unblock things is if I hack up an implementation of the feature myself and work with the owner to refine it.
This is only possible if you have an engineering culture that allows it, but luckily both Google and Microsoft cultures allow this, at least at certain times in the product lifecycle when the tree isn't frozen.
By implementing the feature myself, I'm (a) reducing risk, as we can see the feature sort of works, (b) making it much easier for the overworked feature owner to help me, as they only have to say "change these 3 things and you're good to go", rather than having to take the time to educate me on how to implement the feature, (c) getting a chance to implement the feature exactly the way I want it to work.
Now, I can think of a lot of situations where this approach won't work: at the end of the schedule where no new features are allowed, in projects where the developer is so overloaded that they can't spare any cycles to review the code at all, or in projects where people guard the areas they work on.
But I've been surprised how well it works. And it's getting easier to do, as distributed version control systems become more common, and people become more comfortable working with multiple branches and patches.