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.

Almost 800 people were diagnosed with flu-like symptoms by GPs in the two weeks over Christmas and there have been 195 hospital cases this winter.
This included 25 people in intensive care, while there were 10 outbreaks at hospital wards and care homes.
Pregnant women and those with long-term health conditions are most at risk.
Public Heath Wales (PHW) said the flu strain currently circulating could be particularly serious for older people, but the number of young adults and teenagers visiting GPs with symptoms had also increased.
Dr Richard Roberts, head of the Vaccine Preventable Disease Programme at PHW, said: "We expect the flu virus to circulate at increased levels in Wales for six to eight weeks, with lower levels even longer, so it is important to ensure that remaining eligible people are vaccinated as quickly as possible to help protect them for the rest of this season."
The jab is available from GPs and some pharmacies and is free for those most at risk, including children.
Health and social care workers should also have the vaccination and get can get it through their employers.
Flu is a respiratory illness caused by a virus that affects the lungs and airways and can spread very easily.
Symptoms, which usually last about a week, usually develop very fast and include a high temperature and often include a headache, aching muscles, extreme tiredness and a cough.
Most people with flu symptoms do not need to see a doctor, and PHW advised people to drink plenty of fluids and take ibuprofen or paracetamol to relieve symptoms.

The figure is down from 10 flags awarded last year.
Beaches with a Blue Flag demonstrate their commitment to protecting the coastal environment, water quality, safety, and access for all.
Keep Northern Ireland Beautiful have presented beach and marina operators with 14 national and 10 internationally-recognised awards.
Two marinas have also been awarded blue flags - at Ballyronan, County Londonderry, and Ballycastle, County Antrim.
Tony Wilcox, chairman of Keep Northern Ireland Beautiful, said: "Having Northern Ireland's world class beaches managed to such high standards is helping improve the quality of our lives as well as attracting visitors".
Forty-seven countries participate in awarding Blue Flags to more than 4,000 beaches and marinas worldwide.
The Department of Agriculture, Environment and Rural Affairs monitors identified bathing waters on 20 occasions during the annual bathing season, which runs from the 1 June until 15 September.
In 2016, Portstewart Strand in County Londonderry was awarded the accolade of Best Blue Flag Beach in the UK.
County Down: Cranfield West, Murlough, Tyrella
County Antrim: Portrush West Strand, Portrush East Strand
County Londonderry: Portstewart, Benone, Downhill
Twelve beaches were given the Seaside Award - the national standard for beaches across the UK.
This award guarantees a clean, safe, attractive and well-managed coastal environment with varied levels of facilities provided depending on the location of the beach.
Two Northern Ireland beaches have received the Green Coast Award, which recognises an agreement with the local community to protect and promote a natural beach environment, rather than developing visitor infrastructure.
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.
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Vulnerable people have been urged to have the flu jab as the number of people admitted to hospital with the virus in Wales continues to rise.
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Eight Northern Ireland beaches will proudly display international Blue Flags this summer.