A fundamental rule in Korean society is that Koreans should not "reveal their feelings" to strangers, whether Korean or foreigner. What this means concretely:
-Koreans do not excuse themselves when they bump into people in the streets
-Koreans do not greet people they meet for the first time, unless it is ritualized (two beautiful women standing at the entrance of a supermarket and bowing at clients)
-Koreans do not engage in any form of conversation with strangers who do not have an established relationship with them and only say what has to be said (as I argued before, the owner of a business and his clients may consider his relationship established as owner-client, others ignore this relationship, but in any case, a part-time worker will not engage in conversation with clients unless it is the business's policy)
These rules may sound complicated, but to sum up, Koreans don't talk to strangers. You may notice that in a bus, an old man may pull you out of your chair reserved to the elderly without saying a word. You may also notice Koreans pushing you in the subway or crowded areas without saying a word, or with people in the streets bumping into you or stepping on your feet without saying a word to excuse themselves.
Note that Koreans rarely ask for directions in the streets. They only ask for directions when they are inside a car and the person they are asking directions to knows that they will be out of the way within a few seconds. Therefore, Koreans will switch sidewalks if a stranger is looking at them or approaching them in some way. They leave without saying a word when evangelicals approach them to convert them. And they look into different directions when a stranger approaches them with a question, though they will feel forced to answer if eye contact was established with the stranger.
Koreans use English with foreigners for two reasons: as I argued before, they want the foreigner to know and acknowledge the fact that they are bilingual. They also want to keep the conversation as short as possible, as the foreigner might not understand what the Korean said in Korean, which may lead to a longer conversation. Koreans try to keep conversations short with strangers.
When a third party brings a stranger to a party, the rule is that he must introduce the stranger to the group, doing so loud and clear and giving as much information as possible about the stranger. When introducing a stranger to a group, foreigners often introduce them by saying their names, and then expect the stranger to complete his own self-introduction. Koreans do not communicate with people they do not have enough information about, and not introducing a stranger properly to a group may lead the stranger to be an outcast for the rest of the party. Koreans will avoid looking at the stranger and talking to him.
When a third party introduces a stranger to a group, he usually gives information such as name, age, job, and nationality if it's a foreigner. The third party will then add relevant information regarding why he brought the stranger to the party or how the stranger is in some way affiliated to the group. It is not uncommon for third parties to mention marital status or dating status in introductions.
As some third parties do forget to introduce their guests, it is common that the stranger will ask his third party to introduce him to the group. Once the relationship is established, conversations can go on normally, and Koreans will actively engage in conversation regardless of whether it is a foreigner or a Korean as long as:
-the foreigner observes the local etiquette
-the foreigner speaks at least basic conversational Korean
Note that Koreans refuse to speak languages that may not be understood by some. The reason is that Koreans are highly group oriented and are worried about offending people who may not understand the language. It is therefore important to speak conversational Korean.
While it is considered rude to interrupt a conversation or to comment on what two people are saying during the first meeting, it is important to wait for that conversation to be over and to start the conversation. Starting a conversation includes asking several questions which will determine the relationship between the two and who will lead the conversation. I've given some information on who leads conversations and how relationships are established in other posts.
Koreans also tend to remain neutral during the first meeting and not to give any judgment on anything. They may not laugh at jokes and may not frown on what bothers them. Even while drinking, the newcomer/ stranger will be relatively ignored. Only in subsequent meetings, as the stranger shows commitment to the group, will conversations get more "interesting".
Note finally that Koreans tend to ostracize people who are not "full members of a group". A strong tie to the group is needed, meaning that they should be from the same organization to be considered full members. Therefore, Koreans may not invite strangers who are foreign to the group on a collective basis, but will certainly on an individual basis if a strong relationship is established between two people.