The road closed for a time last week after a landslip during Storm Frank.
It closed again on Monday amid concerns about the boulder, which is about 175m above the carriageway.
The Old Military Road diversion, which was in use since 09:00, closed at 16:30. An alternative diversion via Dalmally will be in place overnight.
In a statement, roads operator Bear Scotland, said that efforts to make safe the boulder had been been "ongoing since first light this morning".
"A helicopter bringing materials to site was delayed reaching the area due to inclement weather but the specialist teams have now begun to install the anchors required to remove the boulder safely," Bear said.
"Good progress has been made and preparatory works should be completed today ready for works to make safe the boulder to take place tomorrow morning."
Bear said the "monitoring process cannot continue safely during the hours of darkness and the Old Military Road will close at 4:30pm this evening and reopen at 9:00am tomorrow".
During this time, the diversion via Dalmally and Crianlarich will be reinstated.
Overnight, the A83 will remain open from Inveraray to the B828 junction, allowing continued access to Lochgoilhead, Dunoon and the ferries to Gourock.
At the south side of the closure, access is available from Tarbet to Ardgartan.
The A83 at the Rest and Be Thankful in Argyll has been closed overnight as efforts continue to make safe a 150 tonne boulder on the hillside.