What's the difference between くる and やってくる?

Question

Can someone explain the nuance between them? They both mean "come", but I'm unsure of when they are interchangeable. My perception is that やってくる seems to put more emphasis on the actual action of coming (compared to the rest of the context, if any) than くる by itself. Compare:

• 友達が来ました
  
  and
  
• 友達がやって来ました

But there seem to be times when you cannot use them interchangeably. For example：

○ さぁ、こっちにこい！　→　OK
? さぁ、こっちにやってこい！　→　Sounds unnatural to me

Will someone please explain where they do and don't overlap?

Answer 1

The dictionary is unfortunately vague on this matter, but some discussion elsewhere on the web lends some clues to the nuance. These discussions say that やってくる, as you suggest, places more emphasis on the act of coming, but especially that the traveler came with some particular effort or purpose, or from especially far away.

From the second link:

「やって来る」は「苦労して、はるばる、遠くから来る」と言うニュアンスを持っているのではと思います。

The writer goes on to list a few examples. If this is the case I think it seems clear why you wouldn't say やってきて to your friend. It almost has kind of an お疲れ様 kind of feel to it. It's just the same way you can't say わざわざ来てください. In fact it may be appropriate to think of やってくる as わざわざ来る, expressing some amount of deference toward the subject, making a command feel unnatural. I can't find any instances on google of it being used as a command. It's like in English you wouldn't issue the command to someone, "Go to the trouble of coming here."

There's also the meaning wherein you can use やってくる to refer to work you've been doing for a long time, like もう２０年この仕事をやってきた。

Answer 2

やってくる is "(something) comes closer (from somewhere)", so it's slightly different from 来る and this gives rise to various nuances.

Your example, 友達がやってきた, gives me the feeling that your friend either showed up unannounced/unexpected, or dilly-dallied on their way. It wasn't a direct and simple "came".

Note that the やってくる you are describing is a unique word (with it's own dictionary entry), so be careful not to confuse it with the grammatical combination of やる + くる, which is also やってくる.