对别人说what someone is doing有很多含义。也许你在试图搭讪，或许你也只是想知道那个人是不是在忙。

Are you busy?
以下对话中what are you doing表示你是不是在忙。

A: Hey, what are you doing?
A：嗨，你干嘛呢？

B: Nothing, why?
B:没干嘛，有事吗？

A: Do you want to come get ice cream with me and Nina?
A:你想和我还有妮娜一起吃冰淇淋吗？

B: Sure! Where should I meet you?
B：当然，我到哪里见你？

In this dialogue, A is asking B what he is doing in order to make sure she can make plans with him. In this kind of situation, you could use these alternatives:
在这段对话中A问B在做什么是为了确保她可以和他一起完成计划。在这种情况下还可以使用以下几个句子：

What are you up to (right now)?
你在忙什么呢？

Are you doing anything (right now)?
你现在在做什么吗？

Are you busy (right now)?
你现在忙吗？

Are you free (right now)?
你现在有空吗？

Do you have any spare time (right now)?
你现在有空余的时间吗？

Do you happen to have any spare time (right now)?
你现在有空余的时间吗？

A: Hey, are you doing anything right now?
A：嗨，你在忙什么呢？

B: Not really, why? (Notice how the response changes here)
B:没做什么，怎么了？（注意这里的回答是怎样转变的）

A: Do you want to come get ice cream with me and Nina?
A: 你想和我还有妮娜一起吃冰淇淋吗？

B: Sure! Where should I meet you?
B: 当然，我到哪里见你？

What is happening?
以下对话中what are you doing表示问对方在做什么。

(Vivian walks into a smoky kitchen and sees Jeff burning food on a pan)
（薇薇走进烟雾缭绕的厨房，看见杰夫在用平底锅做食物）

Vivian: Jeff, what are you doing?
薇薇：杰夫，你干吗呢？

Jeff: Trying to cook. I don’t think it’s working so well.
杰夫：我在做饭，我觉得这个锅不是很好用。

Here, Vivian is asking what Jeff is doing out of shock; she can’t believe what’s happening in front of her. If you’re in Vivian’s position, here are some other reactions you can use:
这里，出于震惊，薇薇问杰夫在做什么，她不敢相信眼前发生的一幕。如果你是薇薇，你也可以用一下这些句子来表达自己的意思。

What’s going on?
这是怎么回事？

What’s going on (in) here?
这是怎么回事？

What is happening (right now)?
发生什么了？

Since this is a strong response, you could also add some phrases like in the world or the hell to these expressions.
这是一种较为强烈的反应，所以你也可以加一些类似于in the world或者 the hell这样的词来加强语气进行表达。

What the heck are you doing?
你在搞什么鬼？

What the hell is going on?
到底怎么回事？

What in the world is happening?
到底发生了什么？

Vivian: Jeff, what is going on in here?
薇薇：杰夫，这是怎么回事？

Jeff: I’m trying to cook. I don’t think it’s working so well.
杰夫：我在做饭，我觉得这个锅不是很好用。