As you type with your iPhone's keyboard, standard plain text characters appear on the screen, but that's not always enough if you're a heavy Emoji user. To insert Emojis, you need to visit the Emoji keyboard and tap on them manually, while the predictive keyboard suggests relevant Emoji from time to time. Nevertheless, it won’t automatically-insert them for you.
If you wish things worked a little differently, then a new free jailbreak tweak called EmojiType by iOS developer Adriano Junior is now available that automatically replaces certain text characters with relevant Emoji characters.