What is the force of the word “behold”?
In the eye of our mind we might imagine the elder with a hand stretched out, pointing. So we may imagine John shifting his gaze to follow the angel's finger to see what he had not noticed before (i.e., a Lamb; see Revelation 5:6).
5 And one of the elders said to me, “Weep no more; behold, the Lion of the tribe of Judah, the Root of David, has conquered, so that he can open the scroll and its seven seals.”