If I have two or more characters in a scene, I generally like to have them talking to each other, unless there are good reasons why they should not be. If they are hiding from their enemies, for example, and need to be quiet.
Or perhaps nothing important to the plot is happening at that point and including the whole conversation only slows down the action. If the characters are discussing where to go for a meal, it's probably enough to say 'They decided to go to the Italian restaurant in the High Street' and pick up the dialogue again when there is some important plot or character development.
I have included dialogue in a novel that might have appeared to be unnecessary, but it was a mystery and there was a clue hidden in the characters' apparently aimless chatter!
Scenes involving more than one character which include big revelations about plot or character should always (in my opinion) be written in the form of dialogue.
I'm not a great planner, but if it's an important scene I usually sketch an outline first, to see how I'm going to get the characters to the point they need to be at the end of it. My outline might look something like this:
Jane - Uncle Matthew was worried about something.
Jane has read Aunt Maria's diary, so she knows it's true (shows Robert the diary).
Robert - Maria was a batty old lady - or words to that effect.
Jane - How dare you talk about her like that! Storms off.
This scene moves things along in several ways. Robert now knows that Jane has the diary. The reader is left in doubt as to whether Aunt Maria is a reliable witness and Jane's concerns are justified. Jane and Robert have parted on bad terms and Jane might be less inclined to confide in him in future.
Once I'm satisfied that the outline takes the scene in the direction I want it to go, I'll write it out in full, establishing the setting, including any of the characters' thoughts or actions that are important, and writing each character's speech in a way that suits his or her personality. If Jane is a confident person, she'll make definitive statements. If she's more hesitant, she might phrase her remarks in the form of questions or say 'I think' and 'perhaps'.
A character who has had little education is likely to have poor grammar and a limited vocabulary. A pompous character might use a lot of long words and speak in convoluted sentences.
The setting will also influence the way characters speak and act. Jane and Robert's conversation will be different depending on whether they are in Aunt Maria's drawing room, on a country walk, in a noisy, crowded pub or at a formal dinner.
What is most important is that all characters should have their own distinctive voices or ways of speaking. Even if Jane Austen did not tell us who was speaking, the reader would never mistake Mrs Bennet's speech for that of Elizabeth or Lady Catherine de Bourgh.