@ChiMi
There is much going on in the main story, but so is everywhere else. But I don't care about things that happen on another continent that will impact the story 2 or 5 million words away.
As others have noted, if an 'interlude' interrupts the story and takes it to somewhere else, with a completely unknown character, that's not an interlude, it's a completely different story. Maybe the author got bored and decided to try his hand at something else but wanted to keep to his regular posting schedule?
The whole purpose of such 'interludes', as others have noted, is to introduce other information into the story that will have relevance later. In other words, it's supposed to enhance the story, and help establish the main characters' character (so you'll understand their later actions).
If someone introduces an unknown character mid-story, I'd not only recommend you skip those chapters entirely, but question whether the overall story is coherent enough to continue. If the author yanks you OUT of the story, only to stove you into an unrelated story with characters with no (established) relation to the current story, he's only driving readers from his own story.
The way to do this, if it's done at all, is to immediately introduce the other characters relevance to the story, but you don't do it during a crisis point in the tale, typically you do it just after an immediate crisis has been resolved (i.e. during a slow period), to help set up the follow dilemma.
Above all, you ONLY introduce new characters in order to further build your existing characters, not to introduce someone who'll rush in at the last minute and 'save' everyone in the book's conclusion.
If I encountered such a chapter, I'd drop the author a note, informing him that, since I saw NO relevance to the extraneous character, I was skipping the following chapters. If I saw a message like that, I'd immediately start restructuring the entire story, as it's a clear red flag that he (the author) has missed the boat entirely.
That said, foreshadowing is extremely tricky, and difficult to pull off successfully. The 'mysterious character' motif is a common, but largely inefficient strategy. If readers don't have some reason for an entirely new character, they're not likely to CARE who the hell he is, meaning the author has already failed in their technique. That's why quickly establishing a connection is essential (I typically start off such episodes using a third-character, who tells/introduces the main character to the new character, so you immediately understand his relationship to the story (ex: is he suspicious, distrustful, overly protective, defensive or jealous?).
