joyeoni mudaeeseo hwanan cheokhaesseo.

It can be either.

Korean does not use capitalization to distinguish names from common nouns, so 조연이 could mean:

• Jo-yeon
  • subject marker 이
• the supporting actor / the supporting role
  • subject marker 이

You need context, or the translation already given to you, to know which one is meant.

이/가 marks the subject of the sentence.

Here, 조연 ends in a consonant, so the subject marker is 이 rather than 가.

• consonant-ending noun + 이
• vowel-ending noun + 가

So:

• 조연이
• but, for example, 배우가

Using 이/가 gives a fairly neutral subject-marking feel. If the sentence had 조연은, that would add a topic or contrast nuance.

Because -에서 marks the place where an action happens.

In this sentence, the pretending happens on the stage, so 무대에서 is natural.

A very simple contrast:

• 무대에서 연기했어 = acted on stage
• 무대에 올라갔어 = went up onto the stage
• 무대에 있었어 = was on the stage

So:

• -에서 = location of an action
• -에 = destination, location of existence, or time in some cases

화난 comes from 화나다.

In this sentence, 화난 is the adnominal form used before 척. It means something like angry or in an angry state.

So it is not just a simple standalone past tense like got angry. Here it is functioning as a modifier:

• 화나다 → dictionary form
• 화난 → form used before a noun-like element such as 척

That is why you get 화난 척하다.

척하다 means to pretend, to act as if, or to make a show of.

The pattern is often:

• verb/adjective + 척하다 = pretend to do / pretend to be

Examples:

• 자는 척하다 = pretend to be sleeping
• 아는 척하다 = pretend to know
• 괜찮은 척하다 = pretend to be okay
• 화난 척하다 = pretend to be angry

You can think of 척 as expressing a kind of pretense or false appearance.

Usually, yes.

This expression normally tells you that the speaker sees the anger as acted or not genuine.

So the sentence strongly suggests:

• the person looked angry
• but that anger was a performance or a pretense

If the speaker only wanted to say the person seemed angry, without claiming it was fake, they would use a different expression.

Because the sentence is in the casual informal past style.

The base expression is 척하다. In the past tense, that becomes 척했어 in casual speech.

Compare:

• 척했어 = casual informal past
• 척했어요 = polite informal past
• 척했습니다 = formal past

So the sentence is something you might say to a friend, someone younger, or in casual narration.

They are similar, but the focus is a little different.

• 화난 척하다 = pretend to be angry

  • focuses on the state/appearance of being angry

• 화내는 척하다 = pretend to get mad / pretend to show anger

  • focuses more on the behavior of expressing anger

So:

• 화난 척했어 = acted as if they were angry
• 화내는 척했어 = acted as if they were angrily reacting/scolding

Both can work in many situations, but 화난 척하다 is a very natural choice when you mean pretended to be angry.

Yes, to some extent.

Korean word order is fairly flexible because particles help show each word’s role. So this is also possible:

• 무대에서 조연이 화난 척했어

That still means basically the same thing.

However, the neutral flow in your original sentence is very natural:

• 조연이 무대에서 화난 척했어

The main rule that usually stays firm is that the verb comes at the end.

Because 화난 is modifying 척하다, and Korean normally writes that modifier separately from the following word.

So:

• 화난 = modifier
• 척했어 = the 척하다 part in past casual form

This is why it appears as:

• 화난 척했어

not as one single block.