In real life, people commonly shout when speaking with great passion. Sometimes in real dialogue, people stress certain words or phrases without raising their voices. This is how we emphasize when speaking. Now, you need to master the art of emphasizing words when you're writing. There is a standard that authors use. Learn it to be one of them.
What We're Yelling About
There are a lot of different ways to use writing to show emphasis on a particular word or phrase. Sometimes, a simple exclamation point is enough.
"Wow! I didn't know Johnny had a power boat!"
Clearly, the person speaking is doing so with a great deal of enthusiasm. One can infer that they're speaking rather loudly and with some emphasis, because of the exclamation points at the end of each thought. There are other ways to show passion in dialogue.
"She said what about my hair?"
The emphasis on the word what shows that the speaker has put emphasis on this word, saying it more strongly than others. This clearly displays emotion -- perhaps dismay, anger or surprise. Without the emphasis, the question becomes curious instead of passionate:
"She said what about my hair?"
And then there are times when people shout -- loudly. To show this, many writers use all caps to put emphasis on the words.
"I told you to GET OUT OF HERE."
The speaker begins to shout when they repeat their instruction. Using all caps is a very effective way to convey a shout, and this is commonly what's used to express shouting, but you don't want to over-do it. All capital letters is very distracting; it pulls the eye and that takes away from all the rest of your words. Use this only in moderation. Stick to italics and exclamation points to show emphasis in situations when an extremely loud shout is not needed nor appropriate.
When it comes to using bold to put emphasis words, just don't do it. This is not the standard in writing. Bold text is used only for headlines and subject lines. In everyday dialogue, bold text just has no place whatsoever.