Hadden Clark is pale and gaunt. His eyes are sunk into their sockets, and his cheeks are almost concave above a gray-flecked goatee. After seven years in prison for the 1992 murder of a young Bethesda woman, he exhibits what one police officer has described as a "skeletal look."

When people enter the Rockville courtroom where Clark is now on trial on a murder charge in the disappearance of a little girl, his gaze follows them from the moment they pass through the door until long after they take their seats. For days, he glared at Edward Tarney, the lead Montgomery County police detective on the case, as he sat in the front row next to the child's mother. When police Lt. Mike Garvey testified, Clark was seen silently mouthing the words "you're lying."

As his attorneys begin today to present their defense against charges that Clark, 47, killed 6-year-old Michele Dorr in 1986, their greatest challenge to persuading the jury may be sitting right beside them.

Rarely has a defendant's mere appearance drawn such a visceral reaction.

Those who have watched the trial over the last two weeks have been whispering not about the strength of the prosecution's case, but about the greatest weakness in the defense: Clark's unnerving appearance in court.

As Neil Jacobs, a Rockville lawyer who sat in on some of the trial last week, observed: "If you'd ever think of your client not being present at a trial, this might be the one to do it in."

A retiree who has begun to watch trials in his spare time said Clark's staring makes him so uncomfortable that he hasn't watched the trial as much as he'd hoped.

"It's a terrible feeling when he looks at you; that's why I'm not in there more," said the man, 58, who declined to be named. "He scares me. If there was ever a murderer, that's what he'd look like."

It's the kind of comment that can rattle a defense lawyer, especially mid-trial.

Clark's attorneys, Assistant Public Defenders Brian Shefferman and Donald Salzman, said they could not comment on their client's demeanor while the trial is underway. However, in opening arguments, Salzman told jurors that police were desperate to solve the 13-year-old case and pinned it on an "emotionally vulnerable" man.

Paul DeWolfe, a Rockville defense lawyer who observed the trial one morning last week, said he interpreted Clark's blank look and malaise as signs of depression, which could inspire sympathy among some jurors. "He certainly doesn't come across to me as a dangerous, angry man," DeWolfe said. "He comes across to me as someone who's ill and not really paying attention because of his mental illness."

However, DeWolfe said Clark could be in trouble if jurors believe "someone has to be deranged" to kill a little girl and hide her body.

Although jurors are instructed to weigh a case strictly on the evidence, defense lawyers say jurors also carefully watch a defendant, perhaps imagining whether the person in front of them could have committed such horrible crimes. In other words, jurors try to picture whether the person fits the part. Many of those observing the trial say Clark seems right out of central casting.

In a case that relies almost entirely on circumstantial evidence, jury experts say, the defendant's appearance and the vibes he sends to jurors are even more important.

"If the evidence is not as clear-cut, the jurors still have to make the same decision--whether he's guilty or not guilty--but they'll have less hard evidence to go on," said Marjorie Fargo, president of Jury Services Inc., an Alexandria-based jury consulting firm.

It's what Fargo calls the "subjective" and "subconscious" part of how jurors reach a decision.

"They'll have to piece it together themselves, and they'll address the issue of whether he could have done it," Fargo said, describing a process that she said she has pieced together by interviewing jurors after they have returned their verdicts.

"They'll think, 'Does he look like he could have done it?' If they play completely by the rules, they shouldn't address that, but human nature takes over, and they do talk about that," Fargo said.

In May 1986, the day Michele vanished, Clark was moving out of the basement of his brother's Silver Spring home, two doors down from the home of Carl Dorr, Michele's father. Prosecutors have argued that Michele, in search of her favorite playmate, Clark's niece, wandered over to the Clark home and, not finding her playmate home, went upstairs to the girl's bedroom to play with dolls while she waited for the girl.

Clark's fellow inmates have testified that Clark told them he discovered Michele in his niece's bedroom and slashed her across the chest and throat with a butcher knife so fiercely that he almost decapitated the child. Her body has never been found.

Six years later, Clark was arrested in the death of Laura Houghteling, 23, who had vanished from her Bethesda home. Clark pleaded guilty to killing Houghteling and pointed out her shallow grave off Old Georgetown Road.

Jurors have been told that Clark is in prison, but not why. Clark's attorneys have argued that Clark clocked into work as a cook at Chevy Chase Country Club the afternoon Michele vanished and that Clark's former fellow inmates concocted the alleged confessions as a bargaining chip with the parole board. They have suggested that Carl Dorr, bitter because of a nasty divorce and child custody fight, had both the motive and the opportunity to kill his daughter.

Last week, Clark appeared bored by the whole courtroom scene. Wearing blue jeans and long-sleeved, button-down shirts, he often seemed to be dozing as prosecutors played a videotape of a police interview with him after he was arrested in September 1998 and charged with killing Michele.

In the interview, Clark mentioned that he was on medication for "a nervous condition" and depression. He denied knowing anything about Michele. On the tape, jurors also saw him sitting alone in the interview room, singing hymns, talking to himself and repeatedly grunting and snorting.

Jacobs noted that jurors also heard testimony that Clark was homeless and lived in a tent in some woods in Bethesda--images that conjure up a recluse.

"There's the stereotype that any homeless person talking to himself is one step away from flying off the handle and killing someone," Jacobs said. "There's that fear. I think that hurts" the defense.

CAPTION: Hadden Clark, already serving time for a 1992 slaying, is accused of killing 6-year-old Silver Spring neighbor Michele Dorr.

CAPTION: Clark is escorted from Montgomery police headquarters after being charged.