the twenty-four elders fall down
Their action is tied to the doxology given by the living creatures. Whenever the creatures shall give glory, the elders shall themselves fall down: πεσοῦνται [pesountai] , future tense middle voice. This is repetitive and voluntary worship (Rev. Rev. 4:10+; Rev. 5:8-10+, Rev. 5:14+; Rev. 11:16+; Rev. 19:4+). They worship at His footstool for He is holy (Ps. Ps. 99:5).