What do you do while you wait? They must have been studying Scriptures, looking through the prophecies, looking for answers, because God knows they had questions! Suddenly, Peter realized what they must do. The prophecies in Psalms had said that someone else should take the betrayer's place. So, they needed to fill up this empty spot.
Acts 1:20 For it is written in the book of Psalms, Let his habitation be desolate, and let no man dwell therein: and his office let another take. Psalm 109:8 Let his days be few; and let another take his office.
How did they fill the empty seat, the empty office, the empty spot in their midst? They spoke, they chose two, and they put them before the Lord, and they prayed. Then they drew lots, like casting dice and drawing for the short straw. They trusted that God would let the correct outcome take place in the random act. Matthias was chosen. Now they were ready. And, so they waited. Look around you. What's missing? Who is missing?