Any release on bail would mark a turn in the campaign for regional elections on December 21, particularly for separatist parties who have repeatedly accused Madrid of taking "political prisoners" and "repression" after their attempt to declare unilateral independence failed.
"State repression is the mobilising element of the independence movement right now," said Oriol Bartomeus, politics professor at the Autonomous University of Barcelona.
"Once they no longer have political prisoners, they will have to change their campaign."
Catalonia's sacked vice-president Oriol Junqueras was the first to be questioned behind closed doors by Llarena - a meeting that lasted just 20 minutes, according to a court source.
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By early afternoon, Llarena had already seen the seven other former regional ministers and was about to question the leaders of two pro-independence civic associations.
Llarena has taken on the investigation into the Catalan leaders, most of whom are accused of rebellion, sedition and misuse of public funds.
However, Llarena of the Supreme Court decided the members of Catalonia's parliament he had been investigating on similar charges could remain free as the probe continues.
Now he has taken on the probe of the other separatist leaders, they hope he will make the same decision for them.
Ester Capella, a national lawmaker for Junqueras's ERC party, said they were ready with the money if the defendants were ordered to post bail.
The jailing of separatist leaders has caused outrage in Catalonia, where pro-independence supporters have organised rallies and wear yellow ribbons as a sign of solidarity.
Today, Catalonia's deposed president Carles Puigdemont, who fled to Belgium after the independence declaration, said he wanted them "home.
"You should get out of prison because you never should have gone there. Do whatever you need to get out," he tweeted.
Puigdemont and several other former regional ministers have remained in Belgium, where they await possible extradition to Spain.
But after the failure of declaring independence, they have not clarified how they will proceed if they win elections again like they did in September 2015 as part of a coalition.
The release of the separatist leaders could also revive tensions within the separatist bloc, particularly between Puigdemont and Junqueras - from a conservative and left-wing party respectively.
Unlike the 2015 elections, Puigdemont's PDeCAT party and Junqueras's ERC will not join forces as part of a coalition and instead are competing to lead the independence movement.
ERC is currently favourite to win the elections later this month according to opinion polls, which also predict separatist and anti-independence parties will be neck-and- neck.